Hemopoiesis in normal adults is restricted to bone marrow (BM) extravascular spaces. Adhesive interactions between hemopoietic cells (HC) and BM stroma are a prerequisite for BM to exert its influence on Hcs and to regulate their migration patterns in and out of the BM. The definition of adhesive interactions between HC and HME are of fundamental importance in development, in normal and abnormal hemopoiesis in adult life and in clinical transplantation and gene therapy. Using in vivo approaches we have provided extensive preliminary data concerning the early phase of cell lodgement/retention within the adult BM in transplantation experiments an its modulation by the disruption of VLA4/VCAM-1 pathway. In this application (1) we will determine homing patterns of fetal cells to fetal livers and fetal BM and compare the results to the ones with adult cells. We will investigate the role of alternative pathways of cytoadhension in development. (2) We will test whether cycling cells have an impaired lodgment to BM compared to quiescent cells. (3) We will study the effects of cytokines on cell lodgement/retention to the BM in experiments that dissociate proliferative effects of cytokines from effects on cytoadhensive behavior. Special emphasis will be given to the effects of kit ligand (KL), using in addition to KL incubation, treatment with anti-kit Ab and studies of lodgment of w/w null fetal liver cells, of normal cells to S1/S1d BM stroma, and of me/me cells with or without KL treatment. (4) We will explore strategies to enhance bone marrow homing in adult transplantation: by studying the effects of VLA4 activation cells; by blocking donor cell sequestration in the liver and divertising more cells to bone marrow; by blocking cell cycle progression in vitro of the cells to be transplanted. (5) We will characterize in detail the homing behavior of human cells into two xenogeneic models (SCID/NOD and fetal sheep). We will test whether perturbation pathways from murine studies are applicable to human cells (anti-VLA4, anti-VCAM-1, fibronectin 30/35, anti- CD44, etc.). By injecting specific populations we will pursue the characterization of cells that give rise to early engraftment in the SCID/NOD model and attempt to improve efficiency in this model. The proposed experiments should answer several biologically significant questions about homing.